Aortic Stenosis (AS) is a progressive valvular disease which has a 2 year survival rate of 50% in symptomatic patients left untreated. Accurate characterization of stenosis severity is crucial in evaluating patients for valve replacement. To determine severity of AS, several clinical indices are typically determined from Doppler flow: these are effective orifice area (EOA), transvalvular pressure gradient (TVPG), ejection time, peak out-flow tract velocity, and peak aortic jet velocity. However, Doppler flow can only derive components of velocities in the direction of insonification and existence of air, bone, or surgical scar present impediments to accurate evaluation. A dilemma that occurs is that in as many as 30% of patients with severe AS who undergo echocardiographic assessment, the indices measured from Doppler are discordant, requiring additional invasive hemodynamic catheterization studies. This may have a physiologic basis but it may also have a root cause in image noise and geometric assumptions. MRI is noninvasive and as with ultrasound, has the capability to image the anatomy as well as velocity and flow. Compared to ultrasound however, the velocities can be measured in all 3D directions and due to its tomorgraphic nature, do not require any geometric assumptions. Furthermore, with the advent of 4D flow imaging, MRI permits 3D visualization of flow as part of a single scan. Despite these advances in MRI, current 4D flow imaging techniques suffer from long scan times and when imaging flow distal to severe stenotic valves are hampered by artifacts. We have addressed these shortcomings through two 4D flow imaging techniques, both adopting non-Cartesian k-space trajectories which significantly reduce echo times (TE): on the one hand, using an Ultra-Short Echo Time (UTE) sequence with radial k-space trajectory (4D UTE Flow) and on the other hand with Spiral k-space trajectory (4D Spiral Flow). The specific aims of the study are as follows: 1) We will construct rigid and distensible flow phantoms of the aortic arch with a stenotic valve. This will permit study of stenotic valvular flows and hemodynamic indices calculated from velocity images at high resolution in idealized and realistic curved geometries. 2) 33 subjects with severe AS as determined by Doppler echocardiography, will undergo a 4D Flow MRI, a Doppler Ultrasound examination, and a hemodynamic study in the cath lab in close succession. From Doppler and MRI all currently adopted clinical indices will be measured and correlated, with cath-based measures as the gold standard. We hypothesize that measures derived from the proposed MRI approaches correlate more closely with cath, alleviating the need for invasive catheterization to resolve discrepant echo findings in many patients. 3) In addition to standard indices, the nature of MRI permits 3D measurement of pressure based on a novel noniterative calculation involving harmonic analysis of pressure gradients, resulting in improved accuracy and speed. This and a new index of flow recirculation, vorticity, will be tested on phantom and patient data.